Optically transmissive materials, such as glass or polymers, may be used as a light guide to propagate light. A light guide typically includes at least one surface adapted to receive light from a light source and an optically smooth surface for reflecting light propagating through the light guide. Common examples of light guides include the optical fibers traditionally used in the data communication industry, and the planar waveguides used in the optical display industry.
Light fibers are also used as components in illumination systems, as disclosed in, for example, U.S. Pat. No. 5,225,166 (Zarian et al.). In these systems, light is injected into at least one end of a light fiber and allowed to exit the fiber at a predetermined position or positions along the length of the fiber.
Methods for encouraging light to exit an optical fiber at a desired location are known as extraction techniques. Many extraction techniques cause light to leak from a light fiber in an uncontrolled fashion. Such techniques include subjecting the fiber to relatively sharp bends (generally known as "microbends") and removing and/or roughening a portion of the optical fiber core or cladding to provide a diffuse surface which causes light to escape.
Extraction techniques are also known which cause light to be extracted from an optical fiber in a controlled fashion. One such technique is disclosed in U.S. Pat. No. 5,432,876 (Appeldorn et al.). There, an illumination device is disclosed which has multiple light extraction structures or notches formed in the core of a light fiber. The extraction structures define first and second reflecting surfaces, which reflect in a radial direction a portion of the light propagating axially along or down the fiber. The reflected light is directed at an angle that is less than the critical angle necessary for continued propagation along the fiber, according to the principle of total internal reflection. As a result, the reflected light is extracted from the fiber.
Various methods of producing extraction structures are known. For example, the extraction structures may be directly micro-machined into the fiber itself. However, one of the drawbacks of this approach is that the materials which have desirable fiber properties (e.g., transparency, flexibility, and high refractive index) are often not amenable to precise micro-machining. Thus, it is very difficult, costly and time-consuming to directly machine features with a microscopic degree of precision and accuracy into the core materials of the fiber.
U.S. Pat. No. 5,631,994 (Appeldorn et al.) is directed toward another method of producing extraction structures. In accordance with this method, an overlay is provided that incorporates the extraction structures. The overlay, which is formed from an optically transparent substrate, is fabricated by conventional manufacturing processes, such as a molding process. An adhesive backing is applied to the overlay so that it can adhere to the fiber core. However, while this method overcomes some of the problems associated with alternative methods of imparting light extracting structures to a light guide, it also requires the presence of two extra interfaces, namely, the fiber/adhesive interface and the adhesive/substrate interface. Each of these additional interfaces can reduce the precision of light extraction, e.g., through undesirable scattering or reflection. Moreover, this method requires that the overlay be carefully aligned along the light guide. Furthermore, the use of an overlay limits the possible arrangement of extraction structures on the light guide. For example, it is difficult to arrange two or more parallel rows of extraction structures in which the structures are offset from one another.
U.S. application Ser. No. 09/026,836, entitled "Method and Apparatus for Seamless Microreplication Using an Expandable Mold," filed Feb. 20, 1998, discloses a method in which the fiber core and extraction structures are formed as an integral unit in a closed mold. While this method avoids the need for an adhesive-backed overlay and has other notable advantages, it also requires the use of an expandable mold. Furthermore, the method described in this application is limited to the use of the same materials for both the core and the light extraction structures. However, the use of diverse materials for the core and light extraction elements is frequently desirable, due to the different processing demands on the two components and the different functions that they serve. For example, as noted above, materials which give good fiber properties are not always amenable to micromachining.
Extrusion methods are known which can be used to create articles, including light guides, in a continuous manner using a thermoplastic feed. However, while extrusion processes are well adapted for the melt processing of thermoplastic materials into profiles that have relatively smooth surfaces, they are less suitable for creating articles that have precision features in a direction transverse to the direction of extrusion, such as the light extraction microstructures in a light guide. This is because, in an extrusion process, the extrudate is still molten and soft as it exits the die, and its final form is affected by cooling and pulling. In the case of a light guide, the percent volume change due to shrinkage as the resin cures typically is large in comparison to the dimensions of the microstructures desired for light extraction. Consequently, the precision of the microstructures is compromised, and they do not perform as intended.
Injection molding may be used to create articles having precision features in a continuous manner. In such a process, molten material is injected under high pressure into a mold which is equipped with the features to be imparted to the finished product. Injection molding is advantageous over extrusion in that both surfaces of the product are controlled during cooling or curing of the thermoplastic material, thereby permitting better control over shrinkage.
Injection molding has been used in various situations in the optical fiber art. Thus, U.S. Pat. No. 4,410,561 (Hart, Jr.) describes the use of injection molding to recoat portions of an optical fiber from which the original coating has been removed during a splicing operation. Japanese Application No. 56123669 (Minoru) makes a somewhat similar disclosure. Other examples of the use of injection molding in this area are described in U.S. Pat. No. 4,531,702 (Plummer), U.S. Pat. No. 5,587,116 (Johnson et al.), U.S. Pat. No. 5,772,932 (Kamiguchi et al.), Japanese Application No. 07296044 (Yasuhiro et al.), Japanese Application No. 06276461 (Yoshiyuki et al.), Japanese Application No. 07006184 (Toshio et al.), Japanese Application No. 06078489 (Takanobu), and Japanese Application No. 06246042 (Tatsuo et al.).
However, injection molding has other limitations that have prevented its use in making optical fibers having precision microstructures thereon. In particular, injection molding requires that the thermoplastic melt be made to take the shape of the mold, and then be allowed to cool. However, the visibly clear, unfilled thermoplastics typically used for optical fibers are poor heat conductors and are subject to significant shrinkage. Therefore, in order to obtain the desired precision while also allowing a sufficiently fast cycle time (usually required to be 60 seconds or less), commercial injection molding of such materials is typically limited to parts having a thickness of less than about 1/4 inch (about 6 mm). The large core optical fibers typically used for illumination purposes are well in excess of these dimensions.
There is thus a need in the art for a continuous process for making large core optical fibers and other light guides which have precision microstructures (e.g., light extractors) on the surface thereof. In particular, there is a need in the art for a continuous process for making light guides which allows for the use of diverse materials for the core and light extraction elements, which avoids the additional interfaces necessitated by the use of adhesive-backed overlay films, which overcomes the difficulties of conventional molding processes (e.g., the adverse effects of shrinkage on precision elements), and which does not require the light extraction structures to be machined into the optical core of the light guide. These and other needs are met by the present invention, as hereinafter described.